Frame and Thread Format

LLDB has a facility to allow users to define the format of the information that generates the descriptions for threads and stack frames. Typically when your program stops at a breakpoint you will get two lines that describes why your thread stopped and where:

* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: test`main at test.c:5

Stack backtraces frames also have a similar information line:

(lldb) thread backtrace
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x0000000100000e85 a.out`main + 4 at test.c:19
    frame #1: 0x0000000100000e40 a.out`start + 52

The two format strings that govern the printing in these output forms can currently be set using the settings set command:

(lldb) settings set thread-stop-format STRING
(lldb) settings set frame-format STRING

The first of these is an abbreviated thread output, that just contains data about the thread, and not the stop frame. It will always get used in situations where the frame output follows immediately, so that information would be redundant. The second is the frame printing.

There is another thread format used for commands like thread list where the thread information isn’t followed by frame info. In that case, it is convenient to have frame zero information in the thread output. That format is set by:

(lldb) settings set thread-format STRING

Format Strings

So what is the format of the format strings? Format strings can contain plain text, control characters and variables that have access to the current program state.

Normal characters are any text that doesn’t contain a {, }, $, or \ character.

Variable names are found in between a ${ prefix, and end with a } suffix. In other words, a variable looks like ${frame.pc}.

Variables

A complete list of currently supported format string variables is listed below:

Variable Name Description
file.basename The current compile unit file basename for the current frame.
file.fullpath The current compile unit file fullpath for the current frame.
language The current compile unit language for the current frame.
frame.index The frame index (0, 1, 2, 3…)
frame.no-debug Evaluates to true if the frame has no debug info.
frame.pc The generic frame register for the program counter.
frame.sp The generic frame register for the stack pointer.
frame.fp The generic frame register for the frame pointer.
frame.flags The generic frame register for the flags register.
frame.reg.NAME Access to any platform specific register by name (replace NAME with the name of the desired register).
function.name The name of the current function or symbol.
function.name-with-args The name of the current function with arguments and values or the symbol name.
function.name-without-args The name of the current function without arguments and values (used to include a function name in-line in the disassembly-format)
function.pc-offset The program counter offset within the current function or symbol
function.addr-offset The offset in bytes of the current function, formatted as ” + dddd”
function.concrete-only-addr-offset-no-padding Similar to function.addr-offset except that there are no spaces in the output (e.g. “+dddd”) and the offset is computed from the nearest concrete function – inlined functions are not included
function.changed Will evaluate to true when the line being formatted is a different symbol context from the previous line (may be used in disassembly-format to print the new function name on a line by itself at the start of a new function). Inlined functions are not considered for this variable
function.initial-function Will evaluate to true if this is the start of the first function, as opposed to a change of functions (may be used in disassembly-format to print the function name for the first function being disassembled)
line.file.basename The line table entry basename to the file for the current line entry in the current frame.
line.file.fullpath The line table entry fullpath to the file for the current line entry in the current frame.
line.number The line table entry line number for the current line entry in the current frame.
line.start-addr The line table entry start address for the current line entry in the current frame.
line.end-addr The line table entry end address for the current line entry in the current frame.
module.file.basename The basename of the current module (shared library or executable)
module.file.fullpath The basename of the current module (shared library or executable)
process.file.basename The basename of the file for the process
process.file.fullpath The fullname of the file for the process
process.id The process ID native to the system on which the inferior runs.
process.name The name of the process at runtime
thread.id The thread identifier for the current thread
thread.index The unique one based thread index ID which is guaranteed to be unique as threads come and go.
thread.name The name of the thread if the target OS supports naming threads
thread.queue The queue name of the thread if the target OS supports dispatch queues
thread.stop-reason A textual reason each thread stopped
thread.return-value The return value of the latest step operation (currently only for step-out.)
thread.completed-expression The expression result for a thread that just finished an interrupted expression evaluation.
target.arch The architecture of the current target
script.target:python_func Use a Python function to generate a piece of textual output
script.process:python_func Use a Python function to generate a piece of textual output
script.thread:python_func Use a Python function to generate a piece of textual output
script.frame:python_func Use a Python function to generate a piece of textual output
current-pc-arrow Prints either -> or `` `` if the current pc value is matched (used in disassembly-format)
addr-file-or-load Formats an address either as a load address, or if process has not yet been launched, as a load address (used in disassembly-format)

Control Characters

Control characters include {, }, and \.

The { and } are used for scoping blocks, and the \ character allows you to desensitize control characters and also emit non-printable characters.

Desensitizing Characters in the Format String

The backslash control character allows your to enter the typical \a, \b, \f, \n, \r, \t, \v, \\, characters and along with the standard octal representation \0123 and hex \xAB characters. This allows you to enter escape characters into your format strings and will allow colorized output for terminals that support color.

Scoping

Many times the information that you might have in your prompt might not be available and you won``t want it to print out if it isn``t valid. To take care of this you can enclose everything that must resolve into a scope. A scope is starts with { and ends with }. For example in order to only display the current frame line table entry basename and line number when the information is available for the current frame:

"{ at {$line.file.basename}:${line.number}}"

Broken down this is:

  • The start the scope: { ,
  • format whose content will only be displayed if all information is available: at {$line.file.basename}:${line.number}
  • end the scope: }

Making the Frame Format

The information that we see when stopped in a frame:

frame #0: 0x0000000100000e85 a.out`main + 4 at test.c:19

can be displayed with the following format:

"frame #${frame.index}: ${frame.pc}{ ${module.file.basename}`${function.name}{${function.pc-offset}}}{ at ${line.file.basename}:${line.number}}\n"

This breaks down to:

  • Always print the frame index and frame PC: frame #${frame.index}: ${frame.pc},
  • only print the module followed by a tick if there is a valid module for the current frame: { ${module.file.basename}`},
  • print the function name with optional offset: {${function.name}{${function.pc-offset}}},
  • print the line info if it is available: { at ${line.file.basename}:${line.number}},
  • then finish off with a newline: \n.

Making Your own Formats

When modifying your own format strings, it is useful to start with the default values for the frame and thread format strings. These can be accessed with the settings show command:

(lldb) settings show thread-format
thread-format (format-string) = "thread #${thread.index}: tid = ${thread.id%tid}{, ${frame.pc}}{ ${module.file.basename}{`${function.name-with-args}{${frame.no-debug}${function.pc-offset}}}}{ at ${line.file.basename}:${line.number}}{, name = '${thread.name}'}{, queue = '${thread.queue}'}{, activity = '${thread.info.activity.name}'}{, ${thread.info.trace_messages} messages}{, stop reason = ${thread.stop-reason}}{\nReturn value: ${thread.return-value}}{\nCompleted expression: ${thread.completed-expression}}\n"
(lldb) settings show frame-format
frame-format (format-string) = "frame #${frame.index}:{ ${frame.no-debug}${frame.pc}}{ ${module.file.basename}{`${function.name-with-args}{${frame.no-debug}${function.pc-offset}}}}{ at ${line.file.basename}:${line.number}}{${function.is-optimized} [opt]}\n"

When making thread formats, you will need surround any of the information that comes from a stack frame with scopes ({ frame-content }) as the thread format doesn’t always want to show frame information. When displaying the backtrace for a thread, we don’t need to duplicate the information for frame zero in the thread information:

(lldb) thread backtrace
thread #1: tid = 0x2e03, stop reason = breakpoint 1.1 2.1
  frame #0: 0x0000000100000e85 a.out`main + 4 at test.c:19
  frame #1: 0x0000000100000e40 a.out`start + 52

The frame related variables are:

  • ${file.*}
  • ${frame.*}
  • ${function.*}
  • ${line.*}
  • ${module.*}

Looking at the default format for the thread, and underlining the frame information:

thread #${thread.index}: tid = ${thread.id}{, ${frame.pc}}{ ${module.file.basename}`${function.name}{${function.pc-offset}}}{, stop reason = ${thread.stop-reason}}{, name = ${thread.name}}{, queue = ${thread.queue}}\n

We can see that all frame information is contained in scopes so that when the thread information is displayed in a context where we only want to show thread information, we can do so.

For both thread and frame formats, you can use ${script.target:python_func}, ${script.process:python_func} and ${script.thread:python_func} (and of course ${script.frame:python_func} for frame formats) In all cases, the signature of python_func is expected to be:

def python_func(object,unused):
  ...
  return string

Where object is an instance of the SB class associated to the keyword you are using.

e.g. Assuming your function looks like:

def thread_printer_func (thread,unused):
  return "Thread %s has %d frames\n" % (thread.name, thread.num_frames)

And you set it up with:

(lldb) settings set thread-format "${script.thread:thread_printer_func}"

you would see output like:

* Thread main has 21 frames